In the course of studying a non-conditional polymerase mutant of B-tropic MuLV we have established the existence of pauses during endogenous reverse transcription and suggested a consensus sequence which may determine pausing by reverse transcriptase. We have located the mutation in the pol gene within a 400-bp region and are currently determining the DNA sequence of this region in the mutant and wild-type viruses. In addition, we have determined the DNA sequence upstream from the transforming gene in two molecularly cloned sarcoma viruses which either do or do not efficiently splice a subgenomic mRNA for the transforming gene. We have shown that the expected splice acceptor has been deleted in the MSV which does not efficiently splice subgenomic mRNA. In addition, sequence alterations have been detected which would account for known alterations in the gag gene product by one of these MSV isolates.